Undesired variation in an integrated circuit chip fabrication process can affect chip performance and manufacturing yield. For example, if one or more physical dimensions of a transistor, such as the width of the gate portion or thickness of an oxide layer, varies from the ideal or as-designed value, the transistor may be unable to operate at the designed-for speed. Such “slow” transistors can hamper chip performance. Slow transistors or other elements are generally localized in regions of the chip (somewhat colloquially referred to as “slow silicon”) that were subjected to undesired process variation. In other words, some areas of a chip may perform better than other areas.
“On-chip variation” (OCV) refers to performance variation among localized areas of a chip as a result of fabrication process factors or operating conditions (e.g., temperature, voltage, etc.). It is desirable to measure OCV so that it can be determined whether the chip is even usable for its intended purpose and, if so, at what speeds it can be reliably operated and under what conditions.
OCV has been measured using ring oscillators that have been included in the chip specifically to measure frequencies, i.e., speeds, at which the chip can operate. A ring oscillator is a well-known device that generally comprises a number of inverters interconnected in a cascaded or ring configuration. A ring oscillator can be used to measure OCV because it oscillates at a frequency that is dependent upon the characteristics and dimensions of the inverters as fabricated (as well as operating conditions such as temperature and voltage). Measuring and comparing the oscillation frequencies or speeds of a number of identically configured ring oscillators located at different areas on the chip can indicate variations among those areas. However, difficulties in using ring oscillators to measure OCV include that a frequency measurement is susceptible to adverse effects of noise and also is not the type of measurement that can be registered readily on a chip and output to external test equipment where it can be analyzed.